This invention is directed to an apparatus and method for detecting detachment of embolic devices and more particularly to detecting detachment of electrically isolated or non-conductive embolic devices.
Numerous intracranial aneurysms rupture each year in North America. The primary purpose of treatment for a ruptured intracranial aneurysm is to prevent rebleeding. There are a variety of approaches to treat ruptured and non-ruptured aneurysms including, for example, delivering an embolic device through an endovascular catheter. In this approach, the interior of the aneurysm is entered by use of a catheter such as those shown in Engelson (Catheter Guidewire), U.S. Pat. No. 4,884,575 and also in Engelson (Catheter for Guidewire Tracking), U.S. Pat. No. 4,739,768. These patents describe devices utilizing guidewires and catheters which allow access to an aneurysm from remote portions of the body. Specifically, by the use of catheters having very flexible distal regions and guidewires which are steerable to the region of the aneurysm, embolic devices which may be delivered through the catheter are an alternative to the extravascular and extra-intravascular approaches.
The endovascular approach typically includes two major steps. The first step involves the introduction of the catheter to the aneurysm site using devices such as shown in the Engelson patents. The second step often involves filling the aneurysm in some fashion or another. For instance, a balloon may be introduced into the aneurysm from the distal portion of the catheter where it is inflated, detached, and left to occlude the aneurysm. In this way, the parent artery is preserved. Balloons are becoming less in favor because of difficulty in introducing the balloon into the aneurysm sac, the possibility of an aneurysm rupture due to overinflation of the balloon within the aneurysm or due to stress placed on the nonspherically shaped aneurysm by the spherical balloon, and the risk associated with traction produced when detaching the balloon.
A highly desirable embolism-forming device that may be introduced into an aneurysm using endovascular placement procedures, is found in U.S. Pat. No. 4,994,069, to Ritchart et al. The devicexe2x80x94typically a platinum/tungsten alloy coil having a very small diameterxe2x80x94may be introduced into an aneurysm through a catheter such as those described in Engelson above. These coils are often made of wire having a diameter of 2-6 mils. The coil diameter may be 10-30 mils. These soft, flexible coils may be of any length desirable and appropriate for the site to be occluded. For instance, the coils may be used to fill a berry aneurysm. Within a short period of time after the filling of the aneurysm with the embolic device, a thrombus forms in the aneurysm and is shortly thereafter complemented with a collagenous material which significantly lessens the potential for aneurysm rupture.
Coils such as seen in Ritchart et al. may be delivered to the vasculature site in a variety of ways including, e.g., mechanically detaching them from the delivery device as is shown in U.S. Pat. No. 5,250,071, to Palermo or by electrolytic detachment as is shown in Guglielmi et al. (U.S. Pat. No. 5,122,136), discussed below.
Guglielmi et al. shows an embolism-forming device and procedure for using that device. Specifically, the Guglielmi device fills a vascular cavity (such as an aneurysm) with an embolic device, typically a platinum coil, that has been endovascularly delivered. The coil is then severed from its insertion tool by the application of a small electric current. Desirably, the insertion device involves a guidewire which is attached at its distal end to the embolic device by a sacrificial joint that is electrolytically dissolvable. Guglielmi et al. suggests that when the embolic device is a platinum coil, the platinum coil may be 1-50 cm. or longer as is necessary. Proximal of the embolic coil is a guidewire, often stainless steel in construction. The guidewire is used to push the platinum embolic coil, obviously with great gentleness, into the vascular site to be occluded. The patent shows a variety of ways of linking the embolic coil to the pusher guidewire. For instance, the guidewire is tapered at its distal end and the distal tip of the guidewire is soldered into the proximal end of the embolic coil. Additionally, a stainless steel coil is wrapped coaxially about the distal tapered portion of the guidewire to provide column strength to the guidewire. This coaxial stainless steel wire is joined both to the guidewire and to the embolic coil. Insulation may be used to cover a portion of the strength-providing stainless steel coil. This arrangement provides for two regions which must be electrolytically severed before the embolic coil is severed from the guidewire.
U.S. Pat. No. 5,423,829 to Pham et al. describes a variation of the Guglielmi detachable coil using an improved sacrificial link between the guidewire and the coil. The size of the sacrificial link is limited to allow more precise placement of the embolic device and facile, quick detachment. The focussed electrolysis found at the sacrificial site reduces the overall possibility of occurrence of multiple electrolysis sites and liberation of large particles from those sites.
Previous attempts to detect the detachment of a coil from, for example, a core wire generally involved a direct current (DC) constant current circuit with a DC voltage monitor to measure DC impedance. The circuit generally included a DC constant current power source having its positive terminal coupled to an electrolytically severable joint or sacrificial link via the core wire. DC current supplied to the joint electrolytically dissolves the sacrificial joint, thereby detaching the coil. The negative terminal of the power source typically was coupled to the patient""s skin via a large skin electrode (e.g., a ground pad or needle). Other grounding arrangements include providing an embolic device delivery microcatheter with a cathode that is electrically coupled to the negative terminal of the power source (see U.S. Pat. No. 5,354,295 to Guglielmi et al.). However, the actual moment of detachment of the occlusion device using these schemes may go undetected because detachment of the coil sometimes occurs without a corresponding significant increase in measured DC impedance.
When longer detachment zones are employed, it has been observed that the actual point of detachment may occur anywhere along the zone. Where the erosion via electroysis occurs distally in that zone, the impedance does not always change.
Accordingly, a DC constant current and monitoring scheme that monitors impedance may not detect the precise moment of detachment if the detachment does not occur exactly at the most proximal point on the sacrificial link. Thus, for these variations of the Guglielmi detachable coils, these schemes do not provide the desired repeatability or accuracy in detecting detachment. When detachment goes undetected, one is unable to precisely determine when the system""s power should be shut down or the pusher wire removed. The time required for the procedure may be unintentionally increased. In addition, particles may be liberated into the blood stream after coil detachment has occurred.
U.S. Pat. Nos. 5,643,254 and 5,669,905 to Scheldrup et al. disclose another method for predictably determining the instant of electrolytic detachment of an embolic device. According to these patents, DC power with alternating current (AC) superposition or modulation is supplied to a sacrificial link or joint. The AC impedance (as measured by the amplitude of the superimposed AC signal) is monitored. When a predetermined change in monitored AC impedance occurs, indicating coil detachment, the DC power is interrupted to minimize or avoid further electrolysis.
With the advent of new types of electrolytically detachable embolic assemblies in which the occlusive device is electrically isolated or non-conductive such as those described in U.S. Pat. No. 5,984,929 to Bashiri et al., the entirety of which are hereby incorporated by reference, these known detection systems can be inadequate. For example, if the detachment zone comes into contact with the electrically isolated coil or a previously detached coil, or if the electrically isolated coil folds back on itself and contacts the detachment zone, the amplitude or signal level corresponding to the superimposed AC impedance will drop to a clearly lower value. While the drop in amplitude does not present a problem in and of itself, if this unintended contact is broken, the subsequent increase in AC impedance, and hence, monitored AC signal level may be interpreted as coil detachment even though the coil may still be attached.
Although an operator may become facile with using such detection devices in connection with non-conductive or electrically isolated coils, if for some reason the operator is unaware or unsure if the coil being detached is electrically isolated or of a previous generation electrically conductive vintage, it may be difficult or impossible for the operator to tell when coil detachment takes place.
Thus, there is a need for a system or assembly that accurately detects detachment of various occlusion devices including electrically isolated or non-conductive occlusion devices.
In addition, previous generation detection systems or assemblies are integrated with power supply controllers incapable of providing additional safeguards to ensure that the patient is not risk. For example, previous generation power supply controllers lack various features including: automatic calibration of the current levels, independent AC and DC circuitry, automatic reduction in DC current, automatic current ramp up, and automatic data storage during detachment procedures.
Thus, there is still a need for a power supply controller that overcomes these and other shortcomings in detecting the detachment of embolic devices.
This invention is an apparatus and method for detecting the detachment of implants which have been introduced to and are intended to remain at the desired therapeutic site. The detachment detection assembly can include a conductive or an electrically-isolated implant, such as a vasoocclusive coil, connected to a wire by an electrolytically severable joint. The detachment assembly further includes a power supply having a direct current drive circuit for supplying a direct current to the joint, an alternating current drive circuit for independently supplying an alternating current to the joint, and an alternating current monitoring circuit for monitoring the level of alternating current supplied to the joint. The detection assembly includes a central processing unit for selecting a first mode and a second mode for determining a detachment threshold level indicating detachment of the implant.
Additionally, a direct current monitoring circuit is provided for measuring an output voltage of the direct current drive circuit wherein a percentage increase in the output voltage indicates detachment of the implant if the alternating current level is greater than the detachment threshold level.